Wavelength multi/demultiplexer

ABSTRACT

Disclosed is a wavelength multi/demultiplexer for separating two wavelength bands with a narrow wavelength spacing. A dielectric multilayer filter is provided in an intersection portion where two optical waveguides intersect each other and separates incident light to the dielectric multilayer filter to transmitted light and reflected light. Here, the distance X from the multilayer surface on the light-incident side of the dielectric multilayer to the central intersection point of the two intersecting optical waveguides is arranged to satisfy 0≦X≦d/2 (where “d” represents the thickness of the dielectric multilayer). With this configuration, a multi/demultiplexer can be realized that shows good wavelength response without spectral degradation even for two wavelengths having narrow wavelength spacing.

TECHNICAL FIELD

The present invention relates to a wavelength multi/demultiplexer usedin an optical communication, and particularly to an optical wavelengthmulti/demultiplexer capable of separating two wavelength bands having anarrow wavelength spacing, with a simple configuration.

BACKGROUND ART

An optical wavelength division multiplexing (WDM) system fortransmitting light of multiple wavelengths through a single transmissionline has been used to realize high-capacity transmission and/orsimultaneous bidirectional transmission. In a WDM system, there are avariety of multi/demultiplexers for combining and/or separatingmultiplexed light, and a low-cost device is required for suchmulti/demultiplexers used in subscriber (access) systems.

FIG. 14 shows a conventional wavelength multi/demultiplexer 500.

The conventional wavelength multi/demultiplexer 500 is a low-costdevice, which combines and/or separates two wavelengths of 1.3 μm and1.55 μm (see Patent Document 1, for example). The “wavelengthmulti/demultiplexer” for use in optical communications is a device thatcombines signals with different wavelengths together and/or separatesthem apart.

The conventional wavelength multi/demultiplexer 500 has single-modeoptical waveguides 2, 3 and 2′, a groove 4 provided at a position wherethe optical waveguides 2 and 3 intersect each other, and a dielectricmultilayer filter 5 inserted into the groove 4. The filter 5 has areflection band at the 1.55 μm band and a pass band at the 1.31 μm band.

The dielectric multilayer filter 5 is arranged to be perpendicular tothe bisector of the intersection angle between the optical waveguides 2and 3, and in a manner that its reflective surface is positioned at theintersection point of the optical waveguides 2 and 3.

As such, a geometric reflective structure is provided by the opticalwaveguides 2, 3 and the dielectric multilayer filter 5, and the opticalwaveguide 2′ is arranged for the light passed through the dielectricmultilayer filter 5. In this way, for the multiplexed light of 1.31 μmand 1.55 μm wavelengths that travel through the optical waveguide 2 viaan optical fiber (not shown), the 1.55 μm light is reflected at thedielectric multilayer filter 5 and output to the optical waveguide 3. Atthe same time, the 1.31 μm light is passed through the dielectricmultilayer filter 5 and output to the optical waveguide 2′.

In this configuration, since the optical waveguide 3, into which the1.55 μm light reflected at the dielectric multilayer filter 5 iscoupled, is a single-mode optical waveguide, how the coupling loss is tobe reduced is an important issue. To solve this issue, a settingposition of the dielectric multilayer filter 5, an intersection anglebetween the optical waveguides 2 and 3, and a maker position forhigh-precision groove processing have been optimized, and amulti/demultiplexer with a required loss has been realized (see PatentDocument 1, for example).

For reference's sake, in the conventional wavelength multi/demultiplexer500, the optical waveguide 2′ is branched into a Y-shape, and for eachof the branched optical waveguides, a laser diode or a photodiode isprovided fabricating a transmitter/receiver module.

It should be note that the Y-shape branched optical waveguides, laserdiode, and photodiode are omitted in FIG. 14.

Recently, there have been advances in the diversification of services inthe access systems, and the wavelength spacing to be separated isbecoming narrower. For example, in the PON (Passive Optical Network)system for single-fiber bidirectional communications, the 1480-1580 nmband used for downstream signals are divided into two bands of 1480-1500nm and 1550-1560 nm. It has then been proposed to assign the latter bandto another future service, such as vide delivery (see Non-PatentDocument 1, for example).

According to this prior art embodiment, a demultiplexer for separatingthe 1480-1500 nm band and the 1550-1560 nm band is required to have aperformance that separates the narrowest spacing of two wavelengths of1500 nm and 1550 nm.

Also, as another prior art embodiment, in an optical line testing systemusing a different wavelength from the communication wavelength, the testlight wavelength of 1650 nm is used relative to the upper limitwavelength of 1625 nm in the communication wavelength band (see PatentDocument 2, for example). In this case, it is required to separate thesignal light and the test light which are adjacent with 25 nm.

If a wavelength multi/demultiplexer for the two wavelengths disposed atsuch a narrow wavelength spacing can be realized with a configurationusing a conventional intersectional optical waveguide, it isadvantageous for cost reduction.

When constructing a wavelength multi/demultiplexer based on the aboveconfiguration, due to the incident light to the dielectric multilayerfilter 5 being divergent light, the wavelength response, that is slopeof the transmission spectrum in the wavelength region from a pass bandto a reflection band in the resulting multi/demultiplexingcharacteristics is degraded. Therefore for narrow separation wavelengthspacing, the wavelength response degradation in the pass band can not beignored. Also, it is required to increase the thickness of thedielectric multilayer to narrow the separation spacing, which leads toaffect spectral degradation due to the divergent light furthermore.

FIG. 15 shows a characteristic of the wavelengthmultiplexer/demultiplexer 500 in the above prior art embodiment.

The inventors have studied with their prototypes by setting therefractive index difference of optical waveguides at a practical lowerlimit of about 0.3% and found a spectral degradation, as shown in FIG.15, in the reflection path from the optical waveguide 2 to the opticalwaveguide 3, which hindered the realization of a wavelengthmulti/demultiplexer.

This spectral degradation has a peak P of a minimum loss around the edgewavelength of the reflection band and shows an increase in loss on itslonger-wavelength side, which cannot be explained from thecharacteristics of the dielectric multilayer filter 5.

In addition, in the multi/demultiplexing spectrum, a problem exists inthat the wavelength response around the edge wavelength from the passband to the reflection band may not be good enough.

Accordingly, it is an object of the present invention to provide awavelength multi/demultiplexer with intersectional optical waveguideshaving no spectral degradation and good wavelength response even for twonarrowly spaced wavelengths.

-   Patent Document 1: Japanese Patent Application Laid-open No.    8-190026 (1996)-   Patent Document 2: Japanese Patent Application Laid-open No.    2002-368695-   Non-Patent Document 1: NTT Technical Journal, Vol. 15, No. 1,    January 2003, pp. 24-27

DISCLOSURE OF THE INVENTION

The present invention is directed to a wavelength multi/demultiplexerfor separating two wavelength bands, disposed at less than a certainspacing. A dielectric multilayer filter is provided in an intersectionportion where two optical waveguides intersect each other and incidentlight to the dielectric multilayer filter is separated into transmittedlight and reflected light. Here, the distance X from the dielectricmultilayer filter surface on the light-incident side of the multilayerfilter to the central intersection point of the two intersecting opticalwaveguides is designed to satisfy 0≦X≦d/2 (where “d” represents thethickness of the dielectric multilayer).

More specifically, in the present invention, spectral characteristic inthe reflection path becomes nearly rectangular shape in the range of thedistance X being from 0 to d/2, and reflection loss does not excessivelyincrease on the longer-wavelength side of the edge wavelength of thereflection band. That is, there exists no minimum loss of the peak P,which is outstanding in the prior art embodiment shown in FIG. 15.

Furthermore, in the present invention, the spectral characteristic inthe reflection path become more close to rectangular shape in the rangeof the distance X satisfying d/10≦X≦2d/5, and the separation is improvedagainst wavelengths disposed with narrow wavelength spacing. Moreover,no increase in reflection loss occurs on the longer-wavelength side ofthe edge wavelength.

The wavelength multi/demultiplexer with intersectional opticalwaveguides according to the present invention advantageously provides nospectral degradation and good wavelength response even for two narrowlyspaced wavelengths.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plane view showing a wavelength multi/demultiplexer 100according to a first embodiment of the present invention;

FIG. 1B is a front view showing the wavelength multi/demultiplexer 100according to the first embodiment of the present invention;

FIG. 1C is a right side view showing the, wavelength multi/demultiplexer100 according to the first embodiment of the present invention;

FIG. 2A is an illustration of the positional relationship, in thewavelength multi/demultiplexer 100, where the distance X from amultilayer surface 5 s of a dielectric multilayer filter 5 to anintersection point C1 of optical waveguides 2 and 3 is 0;

FIG. 2B is an illustration of the positional relationship, in thewavelength multi/demultiplexer 100, where the distance X from themultilayer surface 5 s of the dielectric multilayer filter 5 to theintersection point C1 of the optical waveguides 2 and 3 is ranging from0 to d;

FIG. 2C is an illustration of the positional relationship, in thewavelength multi/demultiplexer 100, where the distance X from themultilayer surface 5 s of the dielectric multilayer filter 5 to theintersection point C1 of the optical waveguides 2 and 3 is d;

FIG. 3 is an illustration at the vicinity of the dielectric multilayerfilter 5 (vicinity of the intersection point C1 of the opticalwaveguides) in the wavelength multi/demultiplexer 100;

FIG. 4 is a graph showing the demultiplexing characteristic obtained bythe first embodiment of the present invention;

FIG. 5 is a graph showing the relationship between the distance X fromthe multilayer surface 5 s of the dielectric multilayer filter 5 to theintersection point C1, and the reflection spectrum from the opticalwaveguide 2 to the optical waveguide 3, in the first embodiment of thepresent invention;

FIG. 6 is a graph showing the relationship between the distance X andthe interval of two wavelengths at which the reflection loss from theoptical waveguide 2 to the optical waveguide 3 is 0.7 dB and 20 dBrespectively, in the first embodiment of the present invention;

FIG. 7 is a graph showing the relationship between the distance X andthe reflection loss at the edge wavelength of the reflection band, inthe first embodiment of the present invention;

FIG. 8 is a summary of suitable range for setting distance X when widthW₂ of the optical waveguides at the intersection portion is 8 μm and 20μm, and the intersection angle 2θ is 8, 10 and 12 degrees, in the firstembodiment of the present invention;

FIG. 9 is a summary where the refractive index difference of the opticalwaveguides is set at 0.45% and the dielectric multilayer filter 5 isreplaced with a dielectric multilayer film of SiO₂ and Ta₂O₅ (with athickness of about 40 μm), whose edge wavelength of pass band is set ataround 1620 mm, in the first embodiment of the present invention;

FIG. 10 is a graph showing the relationship between the distance X andthe reflection loss at the edge wavelength of the reflection band whenthe thickness of the dielectric multilayer 5 is reduced to 25 μm, in thefirst embodiment of the present invention;

FIG. 11 is a graph showing the return loss with respect to theintersection angle 2θ between the optical waveguides, as a parameter ofenlarged width W₂ of the optical waveguides, in the first embodiment ofthe present invention;

FIG. 12 is a graph comparing the reflection spectrum between cases with(solid line) and without (dashed line) the optical waveguide taperedstructure, as a parameter of intersection angle, in the first embodimentof the present invention;

FIG. 13 is an illustration of a wavelength multi/demultiplexer 200according to a second embodiment of the present invention, which showsthe vicinity of a dielectric multilayer film 5 (vicinity of anintersection point C1 of optical waveguides);

FIG. 14 is an illustration of a conventional wavelengthmulti/demultiplexer 500; and

FIG. 15 is a graph showing a characteristic of the conventionalwavelength multi/demultiplexer 500.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention is as in the followingembodiments.

First Embodiment

FIG. 1A is a plane view showing a wavelength multi/demultiplexer 100according to the first embodiment of the present invention, and FIGS. 1Band 1C are, respectively, a front view and a right side view thereof.

FIG. 2A is an illustration of a positional relationship, in thewavelength multi/demultiplexer 100, where the distance X from amultilayer surface 5 s of a dielectric multilayer filter 5 to anintersection point C1 of optical waveguides 2 and 3 is 0. Furthermore,FIGS. 2B and 2C are illustrations of positional relationships, showingrespectively where the distance X is ranging from 0 to d and thedistance X is d, respectively.

The wavelength multi/demultiplexer 100 comprises a silicon substrate 1,single-mode optical waveguides 2, 3 and 2′, a groove 4, and a dielectricmultilayer filter 5.

The single-mode optical waveguides 2, 3 and 2′ comprise a core and aclad made of silica-based glass. The dielectric multilayer filter 5 isplaced within the groove 4.

The optical waveguides 2 and 3 form an intersectional optical waveguidecomprising an intersection point C1 at the central portion of thesubstrate 1, and the reflected light at the dielectric multilayer filter5 is guided into the optical waveguide 3. Also, the optical axis of theoptical waveguide 2′ is aligned with that of the optical waveguide 2 toguide the transmitted light through the dielectric multilayer filter 5into the optical waveguide 2′. The groove 4 is provided where theoptical waveguides 2 and 3 intersect each other, into which thedielectric multilayer filter 5 is inserted and fixed with adhesive (notshown).

The dielectric multilayer filter 5 passes shorter wavelengths and has apass band at the wavelength of 1260-1500 nm and a reflection band at thewavelength of 1550-1600 nm, wherein an alternating multilayer of SiO₂and Ta₂O₅ with a thickness of about 30 μm is formed on a polyimidethin-film substrate (substrate 51) with a thickness of about 5 μm.

Accordingly, for light with wavelength bands of 1260-1500 nm and1550-1600 nm input to the optical waveguide 2, the former 1260-1500 nmband light may be passed and coupled into the optical waveguide 2′, andthe latter 1550-1600 nm band light may be reflected and coupled into theoptical waveguide 3.

The dielectric multilayer filter 5 is arranged such that the multilayersurface 5 s faces to the light-incident side, and that the distance Xfrom the multilayer surface 5 s to the intersection point C1 of theoptical waveguides is 6 μm.

As above, the thickness of the dielectric multilayer film 5 is 30 μm andthat of the substrate 51 is 5 μm, so the dielectric multilayer film 5and the substrate 51 are inserted into the groove 4. Therefore, the halfof total thickness of 30 μm+5 μm=35 μm (17.5 μm) is the distance fromthe multilayer surface 5 s to the center of the dielectric multilayerfilter including the substrate 51. Also, since the distance X from themultilayer film surface 5 s to the intersection point C1 of the opticalwaveguides is 6 μm, the distance from the intersection point C1 of theoptical waveguides to the center of the dielectric multilayer filterincluding the substrate 51 is 17.5 μm-6 μm=11.5 μm.

That is, the center of the dielectric multilayer filter including thesubstrate 51 is positioned at a distance of 11.5 μm apart from theintersection point C1 of the optical waveguides, and the groove 4 isarranged to be perpendicular to the perpendicular bisector of theoptical waveguides 2 and 3 and with a wider width than the totalthickness of the dielectric multilayer film 5 by 2 μm to 3 82 m.

In the first embodiment, the groove 4 is formed by a dicing saw and ametal marker is provided as a positioning reference when forming thegroove 4 on the optical waveguide chip (silicon substrate 1), therebyenabling to maintain the distance X from the multilayer surface 5 s tothe intersection point C1 of the optical waveguides within 6 μm±3 μm.

FIG. 2A shows the positional relationship where the distance X from themultilayer surface 5 s to the intersection point C1 of the opticalwaveguides is 0; FIG. 2B shows the positional relationship where thedistance X satisfies 0≦X≦d (where “d” represents the thickness of thedielectric multilayer 5); and FIG. 2C shows the positional relationshipwhere the distance X is equal to the thickness “d” of the dielectricmultilayer 5.

FIG. 3 is an illustration at the vicinity of the dielectric multilayerfilm 5 (vicinity of the intersection point C1 of the optical waveguides)in the wavelength multi/demultiplexer 100.

At the intersection point C1 where the single-mode optical waveguides 2,3 and 2′ intersect each other, the dielectric multilayer filter 5 isprovided.

For the following description, the single-mode optical waveguides 2, 3and 2′ will be referred to as an input optical waveguide 2, an outputoptical waveguide 3, and an output optical waveguide 2′, respectively.

The input optical waveguide 2 is an optical waveguide guiding inputlight; the output optical waveguide 3 is an optical waveguide guidingreflected light from the dielectric multilayer filter 5; and the outputoptical waveguide 2′ is an optical waveguide guiding transmitted lightthrough the dielectric multilayer filter 5.

For a reason to be described later, smaller divergence angle of thelight beam entering the dielectric multilayer filter 5 is better, andtherefore the refractive index difference of the optical waveguides islimited to about 0.3% to 0.45% and the width of the optical waveguidesis enlarged at the area near the groove 4 to increase the mode fielddiameter.

That is, the input optical waveguide 2 guiding input light consists ofan input optical waveguide 2 a, a tapered optical waveguide 2 b, and anenlarged optical waveguide 2 c. In other words, the optical waveguidewidth of the input optical waveguide 2 a is enlarged through the taperedoptical waveguide 2 b and connected to the enlarged optical waveguide 2c.

The output optical waveguide 2′ consists of an output optical waveguide2′a, a tapered optical waveguide 2′b, and an enlarged optical waveguide2′c. Again, the optical waveguide width of the output optical waveguide2′a is enlarged through the tapered optical waveguide 2′b and connectedto the enlarged optical waveguide 2′c.

Then, so as to secure optical coupling with the input optical waveguide2, the output optical waveguide 2′a, the tapered optical waveguide 2′band the enlarged optical waveguide 2′c are arranged in the pointsymmetric position with respect to the input optical waveguide 2 a, thetapered optical waveguide 2 b and the enlarged optical waveguide 2 c,respectively.

The output optical waveguide 3 consists of an output optical waveguide 3a, a tapered optical waveguide 3 b, and an enlarged optical waveguide 3c. Again, the optical waveguide width of the output optical waveguide 3a is enlarged through the tapered optical waveguide 3 b and connected tothe enlarged optical waveguide 3 c.

Then, so as to secure optical coupling with the input optical waveguide2, the output optical waveguide 3 a, the tapered optical waveguide 3 band the enlarged optical waveguide 3 c are arranged in the mirrorsymmetric position of the input optical waveguide 2 a, the taperedoptical waveguide 2 b and the enlarged optical waveguide 2 c,respectively.

In the above embodiment, the optical waveguides have the refractiveindex difference of 0.3%, and the width of the optical waveguides 2 a, 3a and 2′a at the light input and output terminal portions is 8 μm andenlarged to 25 μm through the tapered optical waveguides 2 b, 3 b and2′b. Furthermore, the optical waveguides 2 and 3 intersect each other atan intersection angle of 12 degrees.

In the above embodiment, in order to stabilize the light mode enlargedthrough the tapered optical waveguide, in the area where the width ofthe optical waveguides is enlarged at around the intersection portionC1, it is preferable to keep the width of the optical waveguidesconstant for a certain length. That is, in the area where the width ofthe optical waveguides is enlarged at around the intersection portionC1, the width of the optical waveguides is preferably a constant up tothe position where the optical waveguides contact each other or theoutside thereof.

The inventors have conducted their experimental studies about the causeof reflection spectral degradation and found that the wavelengthresponse varies significantly depending on the position of thedielectric multilayer filter 5 disposed relative to the intersectionpoint C1 of the optical waveguides.

FIG. 4 is a graph showing the demultiplexing characteristic obtained inaccordance with the first embodiment of the present invention.

For the demultiplexing characteristic obtained by the reflection fromthe optical waveguide 2 to the optical waveguide 3, a flat and low-losscharacteristic is obtained in the longer wavelength than 1550 nm, showsas shown in FIG. 4, and the problem in the prior art, the increased lossin the long wavelength band is solved.

Considering about the case of separating the wavelength bands of1250-1500 nm and 1550-1600 nm in FIG. 4, at the boundary wavelengths of1500 nm and 1550 nm, a good loss characteristic of 1.5 dB or less can beseen for both bands.

While, for an isolation to prevent cross talk from the other side, ithas a sufficient amount of 50 dB or more in the optical waveguides 2 to2′, it is limited to only about 20 dB in the optical waveguides 2 to 3due to pass band ripple of the dielectric multilayer filter 5. This isnot because of the configuration of the wavelength multi/demultiplexer100 in the first embodiment, but is generally seen, such as in aconfiguration of other bulk type wavelength multi/demultiplexer forextracting reflected light from the dielectric multilayer filter 5.

The above characteristic is at sufficient level for practical use evenwhen the wavelength spacing to be separated is further reduced and is 25nm. For example, in the case of demultiplexing the wavelength bands of1250-1515 nm and 1540-1600 nm, loss of 2 dB or less and opticalisolation of 30 dB (in the optical waveguides 2 to 2′) is secured.

FIG. 5 is a graph, in the above embodiment, showing the relationshipbetween the distance X from the multilayer surface 5 s of the dielectricmultilayer filter 5 to the intersection point C1 and the reflectionspectrum from the optical waveguide 2 to the optical waveguide 3.

The above-mentioned “distance X” is the distance from the multilayersurface 5 s on the light-incident side to the point (intersection pointC1) where the center axes of the optical waveguides intersect eachother, as shown in FIGS. 2A to 2C, which will hereinafter be referred toalso as “set distance X.”

In the experiment, the set position of the dielectric multilayer film 5is changed from the position where the intersection point C1 is set atthe multilayer film surface 5 s (distance “X=0” in FIG. 2A) to theposition where the intersection point C1 is set at the boundary of thedielectric multilayer film 5 and the substrate 51 (distance “X=d” inFIG. 2C. The dielectric multilayer filter 5 is a short wavelength passfilter having an alternating multilayer of SiO₂ and Ta₂O₅ with athickness of 30 μm stacked on a polyimide thin-film substrate (substrate51) with a thickness of 5 μm, and its edge wavelength of the stop(reflection) band is set around 1530 nm.

Also, as for the optical waveguides, the relative index difference is0.3%, with a common width W₁ of 8 μm, an enlarged width W₂ of 20 μm, andan intersection angle 2θ of 12 degrees.

As for the example shown in FIG. 5, in the range of the distance X from0 μm to 12 μm, the wavelength response at the edge of the reflectionband is improved as the distance X increases and becomes sharper andcloser to a rectangular shape. However, if the set distance X is furtherincreased and become 15 μm to 30 μm, a peak of the minimum loss aroundthe edge of the reflection band start to appears. Also, on thelonger-wavelength side, the loss tends to increase, and this becomesprominent as the set distance X increases.

The increase in loss on the longer-wavelength side is relatively steep,and it is considered that in this range the coupling of the lightreflected at the dielectric multilayer filter 5 into the opticalwaveguide 3 decreases rapidly. It maybe considered that thecharacteristic (peak P) found in the prior examination as shown in FIG.15 may correspond to this range. One of the reasons for suchcharacteristic is that, though not apparent, much of the reflection fromthe dielectric multilayer filter 5 may be determined with reflectedwaves from around the multilayer surface 5 s, while the wavelengthresponse at around the edge wavelength may be determined with reflectedwaves from the whole multilayer portion.

It is therefore considered that such a phenomenon appears to beoutstanding in a thick multilayer with an increased number of layers toachieve a sharp slope characteristic, and will be the same for not onlya short wavelength pass edge filter used in the present embodiment, butalso a long wavelength pass edge filter and a band-pass filter.

FIG. 6 is a graph showing interval of two wavelengths at which thereflection loss is 0.7 dB and 20 dB, in the reflection spectrum shown inFIG. 5, and indicates that the smaller wavelength interval means thebetter spectral sharpness.

Here, it can be found that the wavelength interval becomes smaller andthe characteristic becomes sharper and closer to a rectangular shape asthe distance X increases in the range of distances X from 0 μm to 12 μmwhile there exists a slight peak at X=12 μm.

Also, assuming to separate two wavelength bands with narrow wavelengthspacing, it is preferable that the set distance X is 3 μm or more.

It should be noted that, for the set distance X from 15 to 30 μm, theloss increases from a peak of the minimum loss around the edgewavelength along towards to the longer-wavelength side and thereflection loss exceeding more than 1 dB is seen (FIG. 5). Therefore,for this range, distance X is not shown in FIG. 6.

FIG. 7 is a graph showing the relationship between the distance X andthe reflection loss at the edge wavelength in the reflection band.

The horizontal axis represents the distance X from the dielectricmultilayer surface to the intersection point of the optical waveguides,while the vertical axis represents the reflection loss at the wavelengthof 1550 nm. In the range of the distance X from 0 μm to 15 μm, thereflection loss does not increase excessively and is 1 dB or less.Furthermore, in the range of the distance X from 3 μm to 12 μm, thereflection loss is at its minimum.

According to the experimental results, if the distance X is set at therange of 0 μm to 15 μm, which corresponds to ½ of the thickness of thedielectric multilayer, the reflection loss does not excessively increasein the reflection band and is within 1 dB. Furthermore, if the distanceX is set at the range between 3 μm and 12 μm, which corresponds to 1/10and ⅖ of the thickness of the dielectric multilayer, respectively, thewavelength response becomes closer to a rectangular shape and theseparation is improved for narrowly disposed wavelengths.

That is, the range of the distance X from 3 μm to 12 μm is best suitedfor achieving both good wavelength response and low reflection loss inthe reflection band. This nature is seen almost similarly in other caseswith different parameters of the intersectional optical waveguide.

In the above embodiment, a primary feature is to control the distance Xfrom the dielectric multilayer 5 to the intersection point C1 of theoptical waveguides within a predetermined range in order to preventreflection spectral degradation which is a problem in the prior artembodiment.

FIG. 8 is a summary for the suitable ranges of the distance X when thewidth W₂ of the optical waveguides in the intersection portion is 8 μmand 20 μm, and the intersection angle 2θ is 8, 10 and 12 degrees.

Here, the condition of 20 μm and 12 degrees corresponds to the case inFIG. 5. It should be noted that the width of the optical waveguides ofW₂=8 μm means that the optical waveguides are not enlarged at theintersection point C1, having a width of 8 μm for the entire length,while the width of the optical waveguides of 20 μm means that the commonwidth of the optical waveguides of 8 μm is enlarged to 20 μm at theintersection point C1.

In FIG. 8, the circular mark indicates that the condition is best suitedfor achieving both good wavelength response and low reflection loss inthe reflection band; the triangular mark indicates that the condition isacceptable for reflection loss; and the cross mark indicates that theset distance X is ill-suited. Regardless of the optical waveguide widthand the intersection angle, the similar results can be obtained as inFIG. 5.

FIG. 9 is a summary for the case where the refractive index differenceof the optical waveguides is set at 0.45% and the dielectric multilayerfilter 5 is replaced with an alternating multilayer of SiO₂ and Ta₂O₅with a thickness of about 40 μm whose edge wavelength of the stop bandis set at around 1620 mm.

The results for the case shown in FIG. 9 are the same as that shown inFIG. 8.

FIG. 10 is a graph showing the relationship between the distance X andthe reflection loss at the edge wavelength of the reflection band whenthe thickness of the dielectric multilayer 5 is reduced to 25 μm.

The horizontal axis represents the distance X from the dielectricmultilayer surface to the intersection point of the optical waveguides,while the vertical axis represents the reflection loss at the wavelengthof 1550 nm. The edge wavelength of the dielectric multilayer filter isset at 1530 nm; the intersection angle 2θ of the optical waveguides is12 degrees; the refractive index difference is 0.3%; and the commonwidth and enlarged width of the optical waveguides is 8 μm and 25 μm,respectively. In the range of the distance X from 0 μm to 12.5 μm, whichcorresponds to ½ of the thickness of the dielectric multilayer, thereflection loss shows good characteristic of 1 dB or less. Furthermore,in the range of the distance X between 2.5 μm and 10 μm, whichcorrespond to 1/10 and ⅖ of the thickness of the dielectric multilayer,respectively, a lower loss characteristic can be obtained.

Summarizing the above results, it is required to put the intersectionpoint C1 of the optical waveguides within the area between the surfaceat light-incident side and the half thickness of the dielectricmultilayer 5 (i.e. 0≦X≦d/2) in order not to increase in the reflectionloss.

In addition, to obtain good characteristics in loss and wavelengthresponse simultaneously, it is preferable, within the above area, tofurther set the distance X within a limited area by about 10% inside ofthe thickness of the dielectric multilayer. More specifically, to obtaingood characteristics in loss and wavelength response simultaneously, itis preferable to satisfy d/10≦X≦2d/5.

In the above embodiment, an example is shown where the distance X fromthe multilayer surface on the light-incident side of the dielectricmultilayer film to the intersection point of the center of the twointersecting optical waveguides is configured to satisfy 0≦X≦d/2 (where“d” represents the thickness of the dielectric multilayer film).

Next, the effect of parameters of optical waveguides onmulti/demultiplexing characteristics will be discussed and then thesuitable parameters of the optical waveguides will be described.

In the wavelength multi/demultiplexer 100 having the dielectricmultilayer filter 5 provided between the optical waveguides, light inputfrom the optical waveguides to the area of the groove 4 and entered thedielectric multilayer filter 5 turns out to be divergent light. Thiswill degrade the characteristics from that of the dielectric multilayerfilter 5 which is designed to be used with collimated light incidence.

In the case of divergent light, the incident angle to the dielectricmultilayer filter 5 spreads by the divergent angle around theintersection angle of the optical waveguides, and light enters thedielectric multilayer filter 5 at different angles. As such, becauseentering the dielectric multilayer filter 5 at different angles willcause the above degradation in wavelength response. This is because thedifferent incident angles cause a slight shift in the wavelengthresponse, and the total transmission spectrum appeared as convolution inwavelength response of the dielectric multilayer filter 5 becomes dullcompared with that with collimated light incidence.

To reduce this effect, it is effective to use optical waveguides with alow refractive index difference and to increase the width of opticalwaveguides contacting to the groove 4. Thus, using optical waveguideswith a low refractive index difference and enlarging the width ofoptical waveguides contacting to the groove 4 enlarges the mode fielddiameter of the optical waveguides contacting to the groove 4 and thisresults in the reduction of the divergence angle of the light enteringthe dielectric multilayer filter 5.

If setting the refractive index difference at less than 0.3%, it willfail to match with the refractive index difference of a standard fiberand tolerable bending radius of the optical waveguides will becomelarger, increasing the size of the optical waveguides. Therefore, it isnot practical to set the relative refractive index difference at lessthan 0.3%. On the other hand, if setting the refractive index differenceat 0.45% or more, it will degrade the wavelength response of thedielectric multilayer filter 5, and the desired separation ofwavelengths can not be obtained.

Accordingly, it is preferable to set the refractive index difference atabout 0.3% to 0.45%.

Also, if the refractive index difference is 0.3% to 0.45%, it ispreferable that the width W₂ Of the enlarged optical waveguide is 18 μmor more relative to the common width W₁ of the input/output opticalwaveguide 2 a of 7 μm to 8 μm. This is because if the width W₂ of theinput/output optical waveguide is less than 18 μm, the obtained effectin enlarging the mode field diameter is small.

Furthermore, while the core thickness of the optical waveguides is setat 7 μm to 10 μm and the cross-section of the input/output opticalwaveguide 2 a is almost rectangular shape, bending loss of the opticalwaveguides can be reduced by setting the core thickness relativelythicker. In this manner, it is advantageous to reduce the curvature ofbent portions to reduce the size of the optical waveguides.

It is preferable to design the length l₁ of the tapered opticalwaveguide 2 b to have the tapered angle of 1 degree or less for oneside, which provides a gentle taper enlarging the mode field diametergradually so that an excess loss can be prevented. Preferably, theenlarged optical waveguide 2 c is extended with a constant width for acertain length, and the length l₂ of the enlarged optical waveguide 2 cis designed to extend longer from the position contacting with the otherintersecting waveguide. By securing a certain length for the length l₂of the enlarged optical waveguide 2 c, propagating mode in the enlargedoptical waveguide 2 c can be stabilized. Thus, the center of the lightinput into the dielectric multilayer filter 5 is aligned with the centerof the optical waveguide, maintaining the reflection characteristicstabilized as well.

The wavelength response at around the edge wavelength of the pass bandalso depends on the intersection angle 2θ between the optical waveguidesshown in FIG. 3. Since the slope of the wavelength response of thedielectric multilayer filter 5 is proportional to cos θ, the larger theincident angle θ to the dielectric multilayer filter 5 is, more salientthe wavelength response degrades around the edge wavelength due to thelight divergence. Therefore, the wavelength response around the edgewavelength depends on the intersection angle 2θ between the opticalwaveguides.

Accordingly, it is preferable to reduce the intersection angle 2θ toavoid degradation of the wavelength response around the edge wavelength.Although reduction in the intersection angle 2θ degrades the return losscharacteristic at the dielectric multilayer filter 5, this degradationcan be alleviated by adopting the enlarged optical waveguideconfiguration.

FIG. 11 is a graph showing, in the above embodiment, the return losswith respect to the intersection angle 2θ between the opticalwaveguides, as a parameter of the enlarged width W₂ of the opticalwaveguides.

The optical waveguides used here have a refractive index difference ofabout 0.3%.

From FIG. 11, although reducing the intersection angle 2θ between theoptical waveguides degrades the return loss characteristic, larger widthW₂ of the optical waveguides leads to larger return loss even with thesame intersection angle 2θ between the optical waveguides. And bysetting the intersection angle between the optical waveguides at 8 to 12degrees and the width W₂ Of the optical waveguides at 20 μm or more,generally good return loss characteristic of more than 35 dB can beobtained. This nature is seen for optical waveguides with a relativerefractive index difference of about 0.45%, and by adjusting the widthW₂ of the optical waveguides the intersection angle between the opticalwaveguides can be set in the range of 8 to 12 degrees.

FIG. 12 is a graph comparing, in the above embodiment, the reflectionspectrum at around the edge wavelength with (solid line) and without(dashed line) the optical waveguide enlarged structure, as a parameterof the intersection angle.

FIG. 12 shows the effect on the wavelength response at around the edgewavelength by introducing the above enlarged optical waveguides andsetting the intersection angle. It can be seen that the slope of thewavelength response at around the edge wavelength is improved in thecase of an intersection angle of 12 degrees compared to 16 degrees. Itcan be also confirmed that the wavelength response can be sharper byenlarging the width of the optical waveguides to 20 μm, while theintersection angle is constant at 12 or 16 degrees. Such improvement onthe wavelength response at around the edge wavelength can be alsoobtained in the pass characteristic from the optical waveguide 2 to theoptical waveguide 2′.

The above embodiment is concerning with a wavelength multi/demultiplexerfor separating two wavelength bands in which edges of the one wavelengthband and of the other wavelength band are disposed with spacing of about50 nm or less. A dielectric multilayer filter is provided in anintersection part where two optical waveguides intersect each other andincident light to the dielectric multilayer filter is separated intotransmitted light and reflected light. Here, the distance X from themultilayer surface on the light-incident side of the dielectricmultilayer to the central intersection point of the two intersectingoptical waveguides is arranged to satisfy 0≦X≦d/2 (where “d” representsthe thickness of the dielectric multilayer film).

Furthermore, the above arrangement of the distance X is effectiveparticularly when the thickness of the dielectric multilayer film 5 is20 μm or more.

Second Embodiment

FIG. 13 is an illustration of a wavelength multi/demultiplexer 200according to the second embodiment of the present invention, and showsthe vicinity of a dielectric multilayer filter 5 (vicinity of anintersection point C1 of optical waveguides).

The configuration of the wavelength multi/demultiplexer 200 is basicallythe same as that of the wavelength multi/demultiplexer 100, except thatan optical waveguide 3′ is provided in the point symmetric position tothe output optical waveguide 3.

The output optical waveguide 3′ consists of an output optical waveguide3′a, a tapered optical waveguide 3′b, and an enlarged optical waveguide3′c. That is, the width of the output optical waveguide 3′a is enlargedthrough the tapered optical waveguide 3′b and connected to the enlargedoptical waveguide 3′c.

In addition, the optical waveguide 3′ may be used as a monitoringterminal, etc., or it may be used as an open terminal.

1. A wavelength multi/demultiplexer comprising a dielectric multilayerfilter at an intersection portion where two optical waveguides intersecteach other and separating incident light to the dielectric multilayerfilter to transmitted light and reflected light, wherein the thickness dof the dielectric multilayer is 20 μm or more and the distance X fromthe multilayer surface on the light-incident side of the dielectricmultilayer to the central intersection point of the two intersectingoptical waveguides satisfies d/10≦X≦2d/5.
 2. The wavelengthmulti/demultiplexer according to claim 1, wherein the width of the twointersecting optical waveguides is enlarged to 18 μm or more toward theintersection portion.
 3. The wavelength multi/demultiplexer according toclaim 2, wherein the enlarged width of the optical waveguides isconstant in the vicinity of the intersection portion.
 4. The wavelengthmulti/demultiplexer according to claim 1, wherein the refractive indexdifference of the optical waveguides is set at 0.3% to 0.45%.
 5. Thewavelength multi/demultiplexer according to claim 1, wherein theintersection angle between said two intersecting optical waveguides is 8to 16 degrees.
 6. The wavelength multi/demultiplexer according to claim2, wherein the refractive index difference of the optical waveguides isset at 0.3% to 0.45%.
 7. The wavelength multi/demultiplexer according toclaim 3, wherein the refractive index difference of the opticalwaveguides is set at 0.3% to 0.45%.
 8. The wavelengthmulti/demultiplexer according to claim 2, wherein the intersection anglebetween said two intersecting optical waveguides is 8 to 16 degrees. 9.The wavelength multi/demultiplexer according to claim 3, wherein theintersection angle between said two intersecting optical waveguides is 8to 16 degrees.